Based on the idea that STZs exist, may be created or destroyed, and can undergo transitions between different states in response to applied shear, Falk and Langer [1]}, [2]} proposed a description of plasticity termed STZ theory. In this description, the population of STZs is described to be governed by an effective or mechanical temperature \(\chi \) . Based on a nonequilibrium thermodynamic formulation of the STZs [3]}, [2]}, expressions for the evolution of the STZ population, the rate of plastic deformation, and the effective temperature itself \(\chi \)  (which evolves during the deformation towards a steady state value governed by the shear rate) have been proposed. Yielding as a function of applied stress arises as a dynamical instability beyond a critical applied stress.
In the original formulation, the STZ theory has no spatial dependence, but it does so in an extended form where the mechanical temperature is treated as varying as a function of space and time [5]}.
These analyses manage to capture many aspects of mechanical response in amorphous solids. An example is shown in Figure REF , which compares computer simulation results of the formation and spreading of shear bands with the predictions of STZ theory[6]}, [5]}. In the mentioned work, the local average potential energies from simulations are compared with the effective temperatures. Although the notion of a mechanical temperature and the comparison with energies are both reasonable and well motivated, there is so far no concrete prescription for measuring this temperature. The other key notion, of an STZ population, with distinct states of occupation, also requires a concrete prescription to directly access it which is currently missing, and has indeed motivated some of the research to identify locations in a glass that are involved in plasticity [6]}, [9]}, [10]}, [11]}.
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